The overwhelming evidence of the images and spectra shows that cooling
does occur at a steady rate over long times (at least several billion
years). Since gas is then cooling out of the hot phase at rates of
hundreds of solar masses per year, an inflow must occur. We do not
expect direct evidence of any inward flow since the velocity is highly
subsonic over most of its volume at v0 6
21/3t10-1/3T8-1/6
km s-1, where =
100 2M
yr-1, t10 is the cooling time
ta at the edge of the flow in
units of 1010 yr, and T = 108T8 K.

The consistency between estimates of
derived from spectral
measurements and those derived from analysis of the surface brightness
is shown in Figure 6. Agreement between these
two methods gives
further support for the existence of cooling flows. The spectral
estimate uses emission lines and blends as a measure of the rate at
which matter is cooling through a given temperature, whereas the
estimate from the image is based on the rate with which matter must be
cooling given its apparent density and temperature profiles.

Figure 6. Total mass deposition rate
obtained from fits to X-ray
spectra (spectral)
compared to those from deprojections of the X-ray
images (image), from
White et al (1991).

The X-ray data also show that the cooling-flow emission is absorbed,
providing evidence for widespread cooled gas. The inferred mass of the
absorbing matter is comparable to that expected from a persistent
cooling flow.